WebExtension.WebExtensionPropertyBag.Append()

I want to populate 'Office.context.document.settings' from DotNet code. Through the XML SDK I can read the settings from document.WebExTaskpanesPart.WebExtensionParts.FirstOrDefault().WebExtension.WebExtensionPropertyBag, but how do I add a 'Setting' ?

WebExtensionPropertyBag is an enumeraltion of OpenXmlElement, so I though WebExtensionPropertyBag.Append() would do it, but OpenXmlElement doesn't support 'new OpenXmlElement()'. So how do you use WebExtensionPropertyBag.Append() ?

Ron.